In a torque converted comprising a lock-up clutch, the control of front-rear differential pressure (lockup differential pressure) of the lock-up clutch involves engagement and disengagement of the lock-up clutch. Because the torque converter makes a transition from a converter state to a lock-up state, the lock-up differential pressure sometimes gradually rises from the prescribed initial differential pressure. In such cases, the torque converter may make a transition from the converter state to the lock-up state via a slip state. In the converter state, the lock-up clutch is disengaged, in the slip state the lock-up clutch slips, and in the lock-up state the lock-up clutch is fully engaged.
Drive slip control that controls the slip rotational speed to a constant value is sometimes conducted in a slip state when a vehicle carrying a torque converter is driven at a constant low speed in a state with open throttle valve. Here, the slip rotational speed is obtained by subtracting the output rotational speed of the torque converter from the engine rotational speed (input rotational speed to the torque converter).
Patent Application Laid-open No. 2003-269603 published by Japanese Patent Office in 2003 discloses the conventional technology for inhibiting the effect of a dead band (or dead zone) of a piston of a lock-up clutch provided in a torque converter. This conventional technology is limited to the case where the operation range of the transmission is neutral and the vehicle speed is almost zero, and it involves pre-charging the piston of the lock-up clutch with oil.